Wild temperature fluctuation
Thread Starter
Newbie
Joined: Sep 2022
Posts: 2
Likes: 0
From: Middletown NJ
G55
Wild temperature fluctuation
Hey guys, first time posting, although Ive lurked for years usually finding the answer to my question via search. No such luck with this one...
Im having some issues with my 2006 G55. this problem has gotten progressively worse (from once in a while, to almost daily) over the past 6 months to the point of not being able to drive the car without it stalling at a red light. Codes P0172 and P0175. The car will hunt for idle and will occasionally die if I dont keep my foot on the throttle to about 800-900RPM. It will also rev up by itself to maybe 2000 RPM. enough to lurch the car forward at a stop light if I dont have my foot firmly on the brake. It has never shut off at speed. I started to notice when this idle fluctuation occurred, the in dash temperature gauge will jump from almost zero to 80/90c, and everything in between very rapidly. It never jumps higher than ~90c though. If this temp gauge fluctuation happens while im driving (part throttle, minimal load) I can feel the car buck. Under partial load I can feel it bog (over fueling?) until the revs start to climb a bit higher then it will clear itself out.
I dont have any codes for temp gauge circuit malfunction/open (unless this will only show up with a STAR computer), so I replaced water pump, thermostat, and sensor last night. Bled the cooling system with a vacuum filler, then let the I/C pump run for a while to bleed, then let it run with the expansion tank cap off. Its 80* here and it sat idling at a rock solid 186*F with the AC on for a good 20 minutes. turn the car off, let it cool down a bit, and let it idle again for another 20 minutes. Idle is perfect, temp is perfect, no gauge fluctuation. Rev it up a bunch, no issue. So I take it for a drive, and I dont make it 100 yards before it starts doing the same fluctuation and idle hunting, bucking, revving. any ideas here???
Truck has 180k miles. Mods & tune have been on for ~5k miles and it ran very strong and trouble free up until now.
77mm upper pulley
S550 throttle body
SLR injectors
Water/meth
MBH long tubes (no cats)
tuned by Tony at RIQ
Im having some issues with my 2006 G55. this problem has gotten progressively worse (from once in a while, to almost daily) over the past 6 months to the point of not being able to drive the car without it stalling at a red light. Codes P0172 and P0175. The car will hunt for idle and will occasionally die if I dont keep my foot on the throttle to about 800-900RPM. It will also rev up by itself to maybe 2000 RPM. enough to lurch the car forward at a stop light if I dont have my foot firmly on the brake. It has never shut off at speed. I started to notice when this idle fluctuation occurred, the in dash temperature gauge will jump from almost zero to 80/90c, and everything in between very rapidly. It never jumps higher than ~90c though. If this temp gauge fluctuation happens while im driving (part throttle, minimal load) I can feel the car buck. Under partial load I can feel it bog (over fueling?) until the revs start to climb a bit higher then it will clear itself out.
I dont have any codes for temp gauge circuit malfunction/open (unless this will only show up with a STAR computer), so I replaced water pump, thermostat, and sensor last night. Bled the cooling system with a vacuum filler, then let the I/C pump run for a while to bleed, then let it run with the expansion tank cap off. Its 80* here and it sat idling at a rock solid 186*F with the AC on for a good 20 minutes. turn the car off, let it cool down a bit, and let it idle again for another 20 minutes. Idle is perfect, temp is perfect, no gauge fluctuation. Rev it up a bunch, no issue. So I take it for a drive, and I dont make it 100 yards before it starts doing the same fluctuation and idle hunting, bucking, revving. any ideas here???
Truck has 180k miles. Mods & tune have been on for ~5k miles and it ran very strong and trouble free up until now.
77mm upper pulley
S550 throttle body
SLR injectors
Water/meth
MBH long tubes (no cats)
tuned by Tony at RIQ
MBWorld Fanatic!
Joined: Jun 2020
Posts: 2,138
Likes: 871
04 E55 AMG (totaled), 07 S550 4Matic, 14 E63S
You need to post the real codes not the generic OBD-II ones. Check fuel trims. A misreporting sensor (or wiring) could cause the mixture to be way off expected. Your codes say too rich (which could happen if the ECU thinks the engine is cold when it's actually fully warmed up).
Thread Starter
Newbie
Joined: Sep 2022
Posts: 2
Likes: 0
From: Middletown NJ
G55
You need to post the real codes not the generic OBD-II ones. Check fuel trims. A misreporting sensor (or wiring) could cause the mixture to be way off expected. Your codes say too rich (which could happen if the ECU thinks the engine is cold when it's actually fully warmed up).
Ill see if I can read short term fuel trims while its acting up, but I suspect youre right with it over fueling thinking its still cold
Last edited by vette427sbc; Sep 9, 2022 at 06:55 PM.
